Spinel is useful for a number of optical, electro-optical and electrical applications including use as a dome, window or port cover for optical instrumentation, such as optical guidance instrumentation in a guided missile or other weaponry. Because of its high optical transparency in ultraviolet, visible and infrared portions of the spectrum, spinel is also useful for electro-optical and electronics applications including use in connection with laser, light-coupled switches, light guides, photo-FET, photo-transistors, photo-diodes, fiber-optic and optical computer applications, and as substrates for electronic circuitry, chip carriers and the like. A number of uses of spinel require that the spinel be bonded or joined to a second material, such as a metallic material. One method useful in achieving such joining is metallization of the spinel, i.e. provision of an adherent metallic layer over at least a portion of the spinel surface. Typically, such metallization layer is used in connection with joining the spinel to a metallic material, such as by brazing, soldering, and similar joining methods. Metallization is also useful for providing conductive pathways for electronics use, including formation of chip carriers, hybrid circuit boards, resistors, capacitors, transistors, and integrated circuits, microwave device substrates, surface acoustic wave devices, resistance temperature devices, and similar electronics-related devices.
Metallization can also be useful in connection with altering physical characteristics of the substrate, such as strength, toughness, abrasion resistance, reflectivity, infrared emission characteristics, and the like.
Most uses for metallization require a relatively strong bond or adherence between the metallization layer and the substrate. In applications where the metallized substrate will be subjected to adverse environments, such as corrosive environments, high temperature environments, high pressure or low pressure environments, and the like, the metallization should remain sufficiently bonded to continue to provide the desired function. In particular, because the coefficient of thermal expansion for spinel is typically different from that for most metals used for metallization purposes, the bonding should be sufficient to withstand any tendency to weaken or loosen the bond or adhesion between the spinel and the metallization layer such as from the stress induced by differential thermal expansion.
Many previous metallization techniques have included treatment of the spinel surface in a manner which would interfere with the optical characteristics of the spinel, such as by providing a roughened or abraded or etched surface. In optics applications, however, it is often necessary to provide the spinel substrate not only in a nonroughened condition but in a highly polished condition, such as a spinel substrate polished to optical quality. Moreover, many workers believed that it was not possible to successfully metallize a single-phase ceramic without using a metal oxide composition and, in particular, believed that it was not possible to metallize with a molybdenum/manganese paste.
A number of metallization techniques require that the substrate be heated to relatively high temperatures. When the metallized spinel is to be employed in optical applications, the metallizing process should not result in sagging, deformation, substantial sublimation or other effects which could interfere with the optical quality of the substrate.
Among the tests which can be used to assist in evaluating the degree of adhesion or bonding of the metallization layer to the spinel, is a pull test. In the pull test, a material, typically a pin, is brazed to the metallization layer and the force, divided by the brazed surface area, which is required to pull the metallized material away from the spinel is measured. Pull test adhesion is typically expressed in terms of pressure, i.e. force per unit area.
Accordingly, there is a need for materials and methods to metallize a high-ultraviolet-transmitting spinel, particularly a spinel which has been polished to optical quality, preferably without interfering with the optical characteristics of the spinel. The metallized spinel should have high pull-test adherence and preferably should be able to withstand high temperatures, such as about 500.degree. C. (ABOUT 1000.degree. F.).